This event has no timer and is pretty straight forward. Once trigged Vendel will follow you until you come across a pack of Burrowing Leapers and a superunique called Armorer's Bane. He'll be standing guard over a Jeweled Scabbard (db) which you simply click on to give to Vendel. He'll be so grateful he'll offer to sell you armor or repair items. You'll need to kill Armorer's Bane and his Scavenger minions however as they'll not be pleased you took their shiny new toy.

The Watch Tower does not always spawn and when it does it's not guarantee that Vendel will appear in it so it may take a little farming to get it and him.